The present application is based on and claims priority from Japanese Patent Application Hei 11-65925 filed on Mar. 12, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a motor having a plurality of permanent-magnet main poles and a plurality of auxiliary poles which are alternately fixed to an inner periphery of a motor yoke.
2. Description of the Related Art
A DC motor disclosed in U.S. Pat. No. 5,723, 929 or its corresponding Japanese application JP-A 9-84315 has a plurality of annularly aligned permanent-magnet main poles and a plurality of auxiliary poles disposed between adjacent two of the main poles. The auxiliary poles are supported by a cylindrical sleeve which is fitted to the inner periphery of the main poles.
The sleeve increases the distance or air gap between an armature and the main poles and reduces the output power of the motor. Usually, such a sleeve is bulged by press-forming against the main and auxiliary poles to make the inside diameter thereof uniform. This may damage permanent-magnets of the main poles and the auxiliary poles.
The present invention has been made in view of the above-stated circumstances.
Therefore, a main object of the invention is to provide a pole arrangement which makes the air gap as small as possible.
Another object of the invention is to provide a pole arrangement in which permanent magnets are immune to damage caused by outside pressure.
According to a main feature of the invention, a pole arrangement of a motor includes a plurality of permanent-magnet main poles disposed circumferentially at equal intervals on the inner periphery of a yoke, a plurality of magnet holders with auxiliary poles therein disposed between the adjacent main poles.
Each magnet holder preferably includes resilient stopper means pressing the main poles in the circumferential direction, and each auxiliary pole is disposed inside the resilient stopper means.
Therefore, the auxiliary poles can be supported by the magnet holders, and additional sleeve is not necessary. As a result, the air gap can be made as small as possible.
Because the pair of resilient stoppers resiliently presses the main poles, each of the magnet holders can be held between two of the main poles by the counteraction thereof. This eliminates additional fixing means such as welding or rivetting.
Each main pole can be held only by the resilient stopper means from circumferentially outside. Therefore, it is not necessary to fix the magnet holders to the inner periphery of the yoke by an adhesive or the like, and the main poles can be assembled easily at a low cost.
According to another feature of the invention, each magnet holder may further include a first stopper member supporting axially opposite ends of the auxiliary pole, a second stopper member supporting axially opposite ends of the main poles, and a third stopper member supporting an outer periphery of the auxiliary poles. Thus, the magnet holder can be formed into a simple U-pipe.
More preferably, the yoke has a pair of projections on an inner periphery thereof, and each magnet holder has a pair of cavities at axially opposite ends which are engaged by the projections so that the magnet holders can be fixed to the yoke in both axial and circumferential directions.